Remove synchronous_replication/max_wal_senders cross-check.

This is no longer necessary, and might result in a situation where the
configuration file is reloaded (and everything seems OK) but a subsequent
restart of the database fails.

Per an observation from Fujii Masao.

src/backend/postmaster/postmaster.c

@@ -735,9 +735,6 @@ PostmasterMain(int argc, char *argv[])
 	if (max_wal_senders > 0 && wal_level == WAL_LEVEL_MINIMAL)
 		ereport(ERROR,
 				(errmsg("WAL streaming (max_wal_senders > 0) requires wal_level \"archive\" or \"hot_standby\"")));
-	if (strlen(SyncRepStandbyNames) > 0 && max_wal_senders == 0)
-		ereport(ERROR,
-				(errmsg("synchronous replication requires WAL streaming (max_wal_senders > 0)")));
 
 	/*
 	 * Other one-time internal sanity checks can go here, if they are fast.